Method of and apparatus for grinding hard materials



Dec. 28, 1965 T. W. ADAMSKI ETAL METHOD OF AND APPARATUS FOR GRINDING HARD MATERIALS Filed Aug. 9, 1962 l IWIIIIIIIIIIIIIIIIIIIIII United States Patent OfiFice 3,226,042 Patented Dec. 28, 1965 11 Claims. 10. 241-40 This invention relates to a method of and apparatus for grin-ding hard materials.

Size reduction is important in the chemical, metallurgical, power-producing, and many other industries. But even though many types of breakers, crushers, and mills exist, no rational method has yet been devised. The process still unproductively consumes large amounts of power, and calls for large and heavy machines and equipment. For the hitherto known disintegrators the efliciency in respect of power is minimal. The physical (energy) efliciency coeflicient of size reduction is calculated to be about 1 percent, and the technical one about 0.1 percent or less, the global consumption of power for size reduction being estimated at 100,000 million kW.-hr.

One of the simplest methods of size reduction is crushing in a crusher between two rotating rolls. The material to be disintegrated is fed between the rolls, which by means of friction, but with considerable slipping, overcome the inertia of the material and accelerate it over a short angular path to a velocity equal to that of the rolls, whereafter it is crushed between the rolls.

An analysis of the process shows that with the existing methods it requires slowly revolving large-diameter rolls. To meet both conditions, correspondingly heavier equipment is needed. The low revolution rates, which in conventional crushers rarely exceed 200 per minute, and the large diameters, of the order of 1500 mm.,

, sharply conflict with the demands of economy in machine building.

It is an object of the invention to provide an improved method of and apparatus for grinding hard materials. In the method of the invention, the rolls perform only the work of crushing, whereas acceleration of the feed is the responsibility of a feeding device. The method enables the phenomenon of slipping to be substantially eliminated.

The invention consists in a method of grinding hard granular material between rolls, which comprises supplying the material to the rolls at a velocity substantially equal to their peripheral velocity and at a rate not exceeding their throughput capacity, the material being supplied in the form of a flat stream having a width not exceeding the working length of the rolls and having a thickness of the order of the grain size of the material before it is ground. By this mode of feeding, which avoids the effects of inertia, the roll crusher is turned into a high-capacity roll mill capable of grinding hard solids at parameters more convenient than those for any of the known mills.

The invention also consists in apparatus for grinding hard granular material, comprising a pair of grinding rolls, means for accelerating the material to reach the rolls at a velocity substantially equal to their peripheral velocity, and means for forming the material, before it is ground, into a flat stream having a width not exceeding the working length of the rolls and having a thickness of the order of the grain size of the material before it is ground. The required acceleration can be produced by the force of gravity when the material feeder is suspended at a suitable height above the rolls. If higher linear velocities are essential, it is convenient to use a stream of fluid (gas or liquid), such as air or another gas, or water or another liquid, which will entrain the feed between the rolls at a suitable linear velocity. Alternatively, one may use a mechanical device that will fling the feed between the rolls, or one may employ a series of roll pairs or sets designed so as to caus the material issuing from between one pair of rolls to acquire the velocity specified for it on reaching the next pair.

Known empirical equations are not applicable to the invented method. Dependent on friction and independent of inertia, the work of the rolls involves exclusively crushing; there is, therefore, theoretically no limit to the rate of revolutions, which makes it possible to use much smaller mills with thin and light rolls. Since the feed enters between the rolls at a velocity close to the peripheral velocity of the latter, it is not abraded by them but only crushed, which reduces wear on the rolls, and also reduces evolution of heat. Since the mill can be many times smaller than any of the currently known pieces of equipment of this kind, material more expensive but of higher quality and greater hardness can be used for the rolls, power consumption can be diminished, and the proportion between the particle size of the feed and that of the disintegrated product can be increased.

One or both rolls may be driven, and their surfaces may be either rough or smooth. The peripheral velocity of the rolls may be at least 5 metres per second, and they may rotate at at least 1500 revolutions per minute.

The roll mill may also incorporate more than two pairs of rolls operating in series and with gaps between the rolls of each pair diminishing in the consecutive pairs, the rate of flow of the material between the rolls of these pairs being suitably synchronised with the peripheral velocity of the successive sets of rolls.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:

FIGURE 1 is a longitudinal section of a grinding apparatus in accordance with the invention;

FIGURE 2 is a plan view of the apparatus of FIG- URE 1 and seen from the plane AA in FIGURE 1; and

FIGURE 3 is a longitudinal section of another form of grinding apparatus in accordance with the invention.

Referring to FIGURES 1 and 2, will be seen that rolls 1, 3 are driven in opposite directions, indicated by arrows (FIGURE 1), by a suitable motor 2 (FIGURE 2). The

distance between the rolls 1, 3 may be adjusted by means of the adjusting device 4. A feeder or hopper 5 is arranged above the rolls 1, 3, and granular material to be ground is placed in the hopper 5 from which it fa ls under the action of gravity down a chute, nozzle or channel 6 to the rolls 1, 3.

The hopper 5 is placed at such a height above the rolls 1, 3 that the material, in falling, acquires by the time it reaches the rolls 1, 3 a velocity approximately equal to their peripheral velocity. The chute, nozzle or channel 6 is of such a shape that the material reaches the rolls 1, 3 in a flat stream having a width not exceeding the working length of the rolls 1, 3, and having a thickness which is of the order of the grain size of the material before it is ground. Slipping and friction between the surfaces of the rolls 1, 3 and the material is thus minimsed, and all the grains of material which are larger than the gap between the rolls are crushed or ground down.

The material should be supplied to the rolls at a rate which does not exceed their throughput capacity, so that the accelerated material is not slowed down or stopped by piling up above the gap between the rolls.

Referring to FIGURE 3, a multi-stage apparatus has three sets of rolls 1, 3 arranged in series and driven by motors 2.

Each stage of the apparatus resembles the apparatus shown in FIGURES l and 2, but the third stage has four pairs of rolls. The granular material is progressively reduced in size as it passes through each stage, and the casing 7 of each chute, nozzle or channel 6 is so shaped that the material reaches each pair of rolls in the form of a hat stream having the above-described characteristics.

By way of example, a roll mill with smooth rolls having the diameter D=60 mm., and speed of revolution n=2600 r.p.m., the gap between the rolls being 6:011 mm., and the working length of the roll I: 60 mm., calls for the material to be fed between the rolls at the linear velocity of v:1r.D.n and 1 :490 metres/ minute This velocity is obtained by suspending the hopper with the material to be ground 340 cm. above the rolls, so that the material falls and acquires in free accelerated motion over the distance of 340 cm. a linear velocity of about 490 metres/ minute.

The theoretical capacity of the mill is:

On substitution of the actual values, V=4.23 cubic metres per 24 hours.

If the bulk density of the feed is 1.60 metric tons per cubic metre, the theoretical capacity of the mill will be 6.8 metric tons per 24 hours. In trials, an actual output of 6.55 metric tons per 24 hours was obtained for quartz sand.

Not only does the method of the invention enable even the hardest materials, such as quartz sand, to be ground well, but it also permits grinding them to a strictly specified grain size.

The material may alternatively be accelerated to the required velocity by means of a mechanical device which flings the material between the rolls, or it may be accelerated by a stream of gas or liquid in which it is entrained.

Since the size of the mill can be many times smal er than the usual, the mill can be conveniently isolated from the environment by a tight housing and can be placed in an atmosphere of inert gas or gases or operated in a vacuum, if the process of grinding gives rise to dusts or injurious gases, or if the material to be ground is sensitive to atmospheric influences or dangerous for the environment.

The literature records rates of revolution not exceeding 1000 per minute, rolls at least well over 10 cm. in diameter, and gaps between the rolls of several millimetres. According to data recorded in literature, a conventional mill having a capacity approaching that given above as an example, would have rolls of the diameter of D=600 mm. at n=800 r.p.m. Such a mill accepts material 0.5 inch in particle size and grinds it to 0.125 inch, i.e., not smaller than 3 mm.

We claim:

1. A method of grinding granular material between a pair of spaced rollers which rotate at the same speed, said method comprising introducing the granular material between the rollers at a speed equal to the peripheral velocity of the rollers and at a rate which is up to and inclusive of the throughput capacity of the rollers which is a function of the spacing between the rollers, the granular material being introduced between the rollers for being ground thereby in the form of a flat stream having a thickness corresponding approximately to the largest grain size of the granular material to be ground and a length corresponding to the length of the rollers.

2. A method as claimed in claim 1, wherein said granular material is introduced to the rollers by being freely dropped from a location above said rollers.

3. A method according to claim 1, comprising accelerating said material to said velocity by the force of gravity.

4. A method according to claim 1, wherein said rollers rotate at at least 1500 revolutions per minute.

5. A method of grinding granular material between a pair of spaced rollers which rotate at the same speed, said method comprising: freely dropping the granular material towards the rollers from a height above the rollers sufficient to cause the material to enter between the rollers for being ground thereby, at a velocity substantially equal to the peripherial velocity of said rollers, the material being dropped at a rate up to and including the throughput capacity of the rollers which capacity is a function of the magnitude of the spacing between the rollers, and restricting the passage of the material to the rollers to cause the material to pass to said rollers in the form of a flat stream having a thickness corresponding approximately to the largest grain size of the granular material to be ground and a length corresponding to the length of the rollers.

6. A method as claimed in claim 5 comprising subsequently grinding the granular material after the same has been ground by the pair of spaced rollers by freely dropping the thus ground material from the pair of rollers towards a second pair of rotating rollers from a height above the second pair of rollers sufiicient to cause the material to enter between the latter rollers, for being ground thereby, at a velocity substantially equal to the peripheral velocity of the latter rollers and restricting the passage of the thus ground material to the second pair of rollers to cause the material to pass thereto in the form of a fiat stream having a thickness corresponding approximately to the grain size of the granular material being discharged from the first set of rollers and a length up to the length of the second pair of rollers.

7. Apparatus for grinding granular material comprising a pair of spaced grinding rollers, means rotating said rollers at the same speed, said rollers having a throughput capacity which is a function of the spacing of the rollers, the diameter of the rollers and the speed of retation of the rollers, means introducing the granular material between the rollers for being ground thereby at a linear speed equal to the peripheral velocity of the rollers and at a rate up to and inclusive of the throughput capacity of the rollers and means for restricting the passage of the granular material to the rollers in the form of a flat stream having a thickness corresponding approximately to the grain size of the granular material to be ground and a length up to the length of the rollers.

8. Apparatus for grinding granular material comprising a pair of spaced horizontal grinding rollers, means rotating said rollers at the same speed, said rollers having a throughput capacity which is a function of the spacing of the rollers, the diameter of the rollers, and the speed of rotation of the rollers, a vertical casing positioned above the space between the grinding rollers, means for introducing granular material to be ground into the casing at a rate which is up to and inclusive of the throughput capacity of the rollers, said casing having an outlet located above the space between the rollers for introducing the granular material into the space for being ground by the rollers, said outlet being dimensioned to cause discharge of the granular material therefrom in the form of a flat stream having a thickness corresponding approximately to the grain size of the granular material to be ground and a length corresponding to the length of the rollers, said means for introducing the material into the casing being located at a particular level above the rollers to cause the material to be discharged at the outlet of the casing under the influence of gravity at a velocity equal to the peripheral velocity of the rollers.

9. Apparatus as claimed in claim 8 comprising an enclosing gas-tight housing.

10. Apparatus as claimed in claim 8 comprising a successive set of horizontally spaced rollers rotating at the same speed located beneath the first mentioned pair of rollers at a distance such that free fall of the material after being discharged from the first pair of rollers will result in the material having a velocity at the level of the successive set of rollers equal to the peripheral velocity thereof, a second casing vertically positioned between the pairs of rollers guiding the passage of material from the first pair of rollers to the second pair of rollers, said second casing having an outlet above the space between the second rollers which is dimensioned to cause discharge therefrom of the ground granular material from the first set of rollers in the form of a fiat stream having a thickness corresponding approximately to the grain size of the thus ground material and a length corresponding to the length of the successive set of rollers.

11. Apparatus as claimed in claim 10 comprising further-successive sets of spaced rotating rollers located beneath the second of the sets of rollers for Subsequently grinding the material discharged therefrom and means for delivering the latter ground material to the successive sets of rollers at linear speeds corresponding to the peripheral speeds of the rollers and in the form of flat streams having thicknesses corresponding to the spacing between the rollers and lengths up to the lengths of the rollers.

References Cited by the Examiner UNITED STATES PATENTS 1,396,712 11/1921 Johnson 24ll59 2,447,258 8/1948 Lobley 24ll59 2,509,148 5/ 1950 Hurst. 2,622,855 12/1952 Kulp et al 241--159 2,986,348 5/1961 Noll et al. 241222 FOREIGN PATENTS 192,406 10/ 1937 Switzerland.

J. SPENCER OVERHOLSER, Primary Examiner. 

1. A METHOD OF GRINDING GRANULAR MATERIAL BETWEEN A PAIR OF SPACED ROLLERS WHICH ROTATE AT THE SAME SPEED, SAID METHOD COMPRISING INTRODUCING THE GRANULAR MATERIAL BETWEEN THE ROLLERS AT A SPEED EQUAL TO THE PERIPHERAL VELOCITY OF THE ROLLERS AND AT A RATE WHICH IS UP TO AND INCLUSIVE OF THE THROUGHPUT CAPACITY OF THE ROLLERS WHICH IS A FUNCTION OF THE SPACING BETWEEN THE ROLLERS, THE GRANULAR MATERIAL BEING INTRODUCED BETWEEN THE ROLLERS FOR BEING GROUND THEREBY IN THE FORM OF A FLAT STREAM HAVING A THICKNESS CORRESPONDING APPROXIMATELY TO THE LARGEST GRAIN SIZE OF THE GRANULAR MATERIAL TO BE GROUND AND A LENGTH CORRESPONDING TO THE LENGTH OF THE ROLLERS.
 7. APPARATUS FOR GRINDING GRANULAR MATERIAL COMPRISING A PAIR OF SPACED GRINDING ROLLERS, MEANS ROTATING SAID ROLLERS AT THE SAME SPEED, SAID ROLLERS HAVING A THROUGHPUT CAPACITY WHICH IS A FUNCTION OF THE SPACING OF THE ROLLERS, THE DIAMETER OF THE ROLLERS AND THE SPEED OF ROTATION OF THE ROLLERS, MEANS INTRODUCTING THE GRANULAR MATERIAL BETWEEN THE ROLLER FOR BEING GROUND THEREBY AT A LINEAR SPEED EQUAL TO THE PERIPHERAL VELOCITY OF THE ROLLERS AND AT A RATE UP TO AND INCLUSIVE OF THE THROUGHPUT CAPACITY OF THE ROLLERS AND MEANS FOR RESTRICTING THE PASSAGE OF THE GRANULAR MATERIAL TO THE ROLLERS IN THE FORM OF A FLAT STREAM HAVING A THICKNESS CORRESPONDING APPROXIMATELY TO THE GRAIN SIZE OF THE GRANULAR MATERIAL TO BE GROUND AND A LENGTH UP TO THE LENGTH OF THE ROLLERS. 